Back throbbing off and on

I've been getting this for about a month or so but the last few days it's really kicked up. My low back will just start to throb, it comes on softly and then builds to a pretty painful sensation and then winds back down. Lasts about a minute. Sometimes correlates with abdominal tightening and sometimes not. Is this the baby pushing on my back? Or a contraction? I've never felt this before. Today it's happened a few times and been almost unbearably painful but it only lasts for like a minute.

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